H2 production from reactions between water and small molybdenum suboxide cluster anions

Electronic Structure of Heteronuclear Cerium-Platinum Clusters

Beyond the now well-known strong catalyst-support interactions reported for ceria-supported platinum catalysts, intermetallic Ce-Pt compounds exhibit fascinating properties such as heavy fermion behavior and magnetic instability. Small heterometallic Ce-Pt clusters, which can provide insights into the local features that govern bulk phenomena, have been less explored. Herein, the anion photoelectron spectra of three small mixed Ce-Pt clusters, Ce2OPt-, Ce2Pt-, and Ce3Pt-, are presented and interpreted with supporting density functional theory calculations. The calculations, which are readily reconciled with the experimental spectra, suggest the presence of numerous close-lying spin states, including states in which the Ce 4f electrons are ferromagnetically coupled or antiferromagnetically coupled. The Pt center is consistently in a nominal -2 charge state in all cluster neutrals and anions, giving the Ce-Pt bond ionic character. Ce-Pt bonds are stronger than Ce-Ce bonds, and the O atom in Ce2OPt- coordinates only with the Ce centers. The energy of the singly occupied Ce-local 4f orbitals relative to the Pt-local orbitals changes with cluster composition. Discussion of the results includes potential implications for Ce-rich intermetallic materials.

Water Gas Shift Reaction Catalyzed by Rhodium-Manganese Oxide Cluster Anions

Fundamental understanding of the nature of active sites in real-life water gas shift (WGS) catalysts that can convert CO and H₂O into CO₂ and H₂ is crucial to engineer related catalysts performing under ambient conditions. Herein, we identified that the WGS reaction can be, in principle, catalyzed by rhodium-manganese oxide clusters Rh₂MnO_1,2^- in the gas phase at room temperature. This is the first example of the construction of such a potential catalysis in cluster science because it is challenging to discover clusters that can abstract the oxygen from H₂O and then supply the anchored oxygen to oxidize CO. The WGS reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemical calculations. The coordinated oxygen in Rh₂MnO_1,2^- is paramount for the generation of an electron-rich Mn⁺-Rh^- bond that is critical to capture and reduce H₂O and giving rise to a polarized Rh⁺-Rh^- bond that functions as the real redox center to drive the WGS reaction.

Lanthanide Oxides: From Diatomics to High-Spin, Strongly Correlated Homo- and Heterometallic Clusters

Small lanthanide (Ln) oxide clusters present both experimental and theoretical challenges because of their partially filled, core-like 4f ⁿ orbitals, a feature that results in a plethora of close-lying and fundamentally similar electronic states. These clusters provide a bottom-up approach toward understanding the electronic structure of defective or doped bulk material but also can offer a challenge to the theorists to find a method robust enough to capture electronic structure patterns that emerge from within the 4f ⁿ (0 < n < 14) series. In this Feature Article, we explore the electronic structures of small lanthanide oxide clusters that deviate from bulk stoichiometry using anion photoelectron spectroscopy and supporting density functional theory calculations. We will describe the evolution of electronic structure with oxidation and how Ln_xO_y^- cluster reactivities can be correlated with specific Ln-local orbital occupancies. These strongly correlated systems offer additional insights into how interactions between electrons and electronically complex neutrals can lead to detachment transitions that lie outside of the sudden one-electron detachment approximation generally assumed in anion photoelectron spectroscopy. With a better understanding of how we can control nominally forbidden transitions to sample an array of spin states, we suggest that more in-depth studies on the magnetic states of these systems can be explored. Extending these studies to other Ln-based materials with hidden magnetic phases, along with sequentially ligated single molecule magnets, could advance current understanding of these systems.

[Mo3 S13 ]2- as a Model System for Hydrogen Evolution Catalysis by MoSx : Probing Protonation Sites in the Gas Phase by Infrared Multiple Photon Dissociation Spectroscopy

Materials based on molybdenum sulfide are known as efficient hydrogen evolution reaction (HER) catalysts. As the binding site for H atoms on molybdenum sulfides for the catalytic process is under debate, [HMo3 S13 ]- is an interesting molecular model system to address this question. Herein, we probe the [HMo3 S13 ]- cluster in the gas phase by coupling Fourier-transform ion-cyclotron-resonance mass spectrometry (FT-ICR MS) with infrared multiple photon dissociation (IRMPD) spectroscopy. Our investigations show one distinct S-H stretching vibration at 2450 cm-1 . Thermochemical arguments based on DFT calculations strongly suggest a terminal disulfide unit as the H adsorption site.

Using anion photoelectron spectroscopy of cluster models to gain insights into mechanisms of catalyst-mediated H2 production from water

Metal oxide cluster models of catalyst materials offer a powerful platform for probing the molecular-scale features and interactions that govern catalysis. This perspective gives an overview of studies implementing the combination of anion photoelectron (PE) spectroscopy and density functional theory calculations toward exploring cluster models of metal oxides and metal-oxide supported Pt that catalytically drive the hydrogen evolution reaction (HER) or the water-gas shift reaction. The utility in the combination of these experimental and computational techniques lies in our ability to unambiguously determine electronic and molecular structures, which can then connect to results of reactivity studies. In particular, we focus on the activity of oxygen vacancies modeled by suboxide clusters, the critical mechanistic step of forming proximal metal hydride and hydroxide groups as a prerequisite for H2 production, and the structural features that lead to trapped dihydroxide groups. The pronounced asymmetric oxidation found in heterometallic group 6 oxides and near-neighbor group 5/group 6 results in higher activity toward water, while group 7/group 6 oxides form very specific stoichiometries that suggest facile regeneration. Studies on the trans-periodic combination of cerium oxide and platinum as a model for ceria supported Pt atoms and nanoparticles reveal striking negative charge accumulation by Pt, which, combined with the ionic conductivity of ceria, suggests a mechanism for the exceptionally high activity of this system towards the water-gas shift reaction.

Effect of Alkyl Group on MxOy(-) + ROH (M = Mo, W; R = Me, Et) Reaction Rates

A systematic comparison of MxOy(-) + ROH (M = Mo vs W; R = Me vs Et) reaction rate coefficients and product distributions combined with results of calculations on weakly bound MxOy(-)·ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous MxOy(-) + H2O reactivity studies. MxOy(-) + ROH → MxOy+1(-) + RH oxidation reactions are observed for the least oxidized clusters, and MxOy(-) + ROH → MxOyROH(-) addition reactions are observed for clusters in intermediate oxidation states, as observed previously in MxOy(-) + H2O reactions. The first step is weakly bound complex formation, the rate of which is governed by the relative stability of the MxOy(-)·ROH charge-dipole complexes and the Lewis acid-base complexes. Calculations predict that MoxOy(-) clusters form more stable Lewis acid-base complexes than WxOy(-), and the stability of EtOH complexes is enhanced relative to MeOH. Consistent with this result, MoxOy(-) + ROH rate coefficients are higher than analogous WxOy(-) clusters. Rate coefficients range from 2.7 × 10(-13) cm(3) s(-1) for W3O8(-) + MeOH to 3.4 × 10(-11) cm(3) s(-1) for Mo2O4(-) + EtOH. Second, a covalently bound complex is formed, and anion photoelectron spectra of the several MxOyROH(-) addition products observed are consistent with hydroxyl-alkoxy structures that are formed readily from the Lewis acid-base complexes. Calculations indicate that addition products are trapped intermediates in the MxOy(-) + ROH → MxOy+1(-) + RH reaction, and the third step is rearrangement of the hydroxyl group to a metal hydride group to facilitate RH release. Trapped intermediates are more prevalent in MoxOy(-) reaction product distributions, indicating that the rate of this step is higher for WxOy+1RH(-) than for MoxOy+1RH(-). This result is consistent with previous computational studies on analogous MxOy(-) + H2O reactions predicting that barriers along the pathway in the rearrangement step are higher for MoxOy(-) reactions than for WxOy(-).

Hydrogen evolution from water using Mo–oxide clusters in the gas phase: DFT modeling of a complete catalytic cycle using a Mo2O4−/Mo2O5− cluster couple

Hydrogen evolution from water using sacrificial reagents and Mo–oxide cluster anions has been explored. The internal energy preservation within the clusters plays a key role in the catalytic cycle.

Reactions of metal cluster anions with inorganic and organic molecules in the gas phase

Progress on the activation and transformation of important inorganic and organic molecules by negatively charged bare metal clusters as well as ligated systems with oxygen, carbon, and nitrogen, among others.